Oct20
2011

By: admin                Categories: 3D Animation

From the hallowed halls of Disney came twelve principles of animation that have shaped contemporary and traditional animation techniques and workflows.  Roland Hess’ Tradigital Blender bridges the gap between the twelve principles of animation and your own digital work in Blender.  Previously, Roland discussed using the classic animation principles Squash & Stretch, Anticipation, and Staging.  Today’s post will cover Overlap and Follow Through, Slow In and Out, and Arcs.

Overlap and Follow Through

Although they are used for different effects, both of these terms demonstrate the physical principle of inertia on the character. Remember Newton’s first law of motion (paraphrased): stuff wants to remain at its current velocity (which could include zero velocity—standing still), unless something happens to it. Kind of obvious, but the implications in animation are immense. Inertia is simply an object’s desire to just keep doing what it is that it was doing before. A block of wood on the ground isn’t going
anywhere by itself. Give it a kick, and it takes off.

Think about the foot that gave the kick for a moment. It was just happily planted on the ground a moment ago, when all of the sudden the hip rotated, which pulled the upper leg along with it, which put torque on the knee joint, which transferred that energy to the lower leg, on again to the foot which finally moved. In real time, it didn’t take long for that chain of motion to reach the leg, only a fraction of a second, really. But each part of the leg experienced inertia and wanted to just stay where it was until something else made it move. If you were to slow down a karate kick with high speed film, the effect would be obvious. At first, the foot and lower leg drag behind the rotating hips and driving quads, only snapping forward at the end like a whip.

FIG 1.8 (a–d) A Leg with Overlapping Action Demonstrates the Inertia of the Body.


Fig. 1.8
shows Junot’s leg doing just this. When you make your characters move this way, it is called overlap. One other aspect of overlap is to make sure that your character’s limbs (and other assorted bits) do not move perfectly in sync with one another. Motion professionals like dancers or magicians may do this, but in general if you reach out to grab something from a counter top with both hands, they will not hit the object at the same time. There will be only a fraction of a second of difference between the two, but in animation, you are responsible for those fractions. Only machines move in perfect synchronicity.

The second part of this principle comes into play after the motive force of the event is over. After the kick, what happens? The character doesn’t simply withdraw its foot and return to a standing position. Weight shifts. Momentum probably carries the body forward into a new stance. If the character has long hair, loose clothing or pockets of fat, it will continue to move after the rest of the body has stopped. This is follow through. If you’ve ever played sports, you’ll have heard your coaches encouraging you to follow through and finish your motion. You need to do that in CG as well.

Slow In and Out

CG animators often use the term “ease in” and “ease out” for this principle. It refers once again to the physical principle of inertia. If something is stopped, it can’t just be moving rapidly in the next instant. It starts out slowly, gaining speed, until it has reached “cruising velocity.” Then, before it stops, it slows down, unless it does something disastrous like smashing into a concrete wall, in which case it stops almost immediately and probably exhibits some extreme squash and stretch. Wiley Coyote, please call your doctor. This was a big deal in the days of hand-drawn animation, and many beginners think that since the computer handles this for you, it’s not relevant any more.

FIG 1.9 Two Balls, Moved in Time (Ball Bouncing along a Floor).

FIG 1.10 Two Balls, Moved in Time (Ball without a Bounce Point).

While it’s true that computer interpolation can generally handle ease in and out, the way that it does so encodes a lot of information for the viewer. Check the very standard “bouncing ball” diagram above in Figs. 1.9 and 1.10.  The Fig. 1.9 demonstrates a standard ease in/out for a moving ball. Note how the more widely spaced balls near the bottom of the figure suggest that the ball is moving rapidly there, while the closely spaced ones near the top of the arc indicate slower vertical motion. It looks like a ball bouncing along a floor. In contrast, Fig. 1.10 shows a different motion path, one without a “bounce” point where our intuition tells us that a floor would belong in the other illustration. It certainly doesn’t look right for a bouncing ball. Perhaps it’s an overhead view of a ball weaving back and forth across a plane. Really though, the only difference between the two motion paths of the ball is the way that slow in/out has been handled. Properly handling this slow in/out is crucial to the perception that your motion is physically correct.

Arcs

FIG 1.11 Smith’s Arm, Moving Only at the Shoulder

This one’s pretty simple, and not as much of a problem in 3D as it is in traditional animation. Fig. 1.11 shows Smith’s arm moving at the shoulder. Note the arc that the fingertips make. It’s pretty obvious why this happens. Your arm is more or less a rigid figure. If you brace the elbow, your arm will be the same length no matter how you position your shoulder. In traditional animation, artists called “In Betweeners” had the job of drawing the frames between the key frames drawn by the lead animators. Figs. 1.12 and 1.13 show two different ways to draw the frame midway between the extreme arm positions. Clearly, Fig. 1.13 is the correct way to do it, but apparently the industry had enough people silly enough to do it the other way that this was worth including among the principles.

FIG 1.12 Tweening Arm Positions the Wrong Way.

FIG 1.13 Tweening Arm Positions the Correct Way, with an Arc.

Tradigital Blender is available at Amazon, Barnes & Noble, and wherever fine books can be found.  The Tradigital series will release three new titles covering Maya, 3ds Max, and Cinema 4D in 2011 and 2012.

Excerpted from Tradigital Blender, by Roland Hess. © 2011, Elsevier, Inc. All rights reserved.

Roland Hess

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